The problem addressed by the disclosure falls within the scope of supplying a linear actuator with hydraulic fluid but is not limited to this application.
In the example in FIG. 1, a fixed case element 1 of an engine, such as a turbine engine, comprises a frustoconical collar 1V and an axial extension 1C on which a linear actuator or linear hydraulic actuator 3 is mounted. The actuator is formed from a static portion 31, forming a piston, which is rigidly connected to the casing axial extension and a portion 32, forming a cylinder, which is movable in translation along the axis of the casing. The static and movable portions delimit two chambers, A and B respectively, therebetween, which chambers are supplied, one by an opening 35A of the chamber A, which is located on the side of the collar, and an opening 35B of the opposing chamber B. The movable portion of the actuator is connected to a member (not shown) of the engine which is used to control the position. The collar 1 comprises an opening 7A for selectively distributing fluid which is connected by a duct for supplying hydraulic fluid to the opening 35A of the chamber A of the actuator and an opening 7B for selectively distributing fluid which is connected by a duct for supplying hydraulic fluid to the opening of the chamber B of the actuator.
In operation, the distance between the openings for distributing fluid in the collar and that of the chambers of the actuator varies according to the position of the movable portion 32, along the axis of the casing extension 1C. A means for adapting the length of the ducts for supplying fluid to the distance between the openings is to arrange telescopic tubular rods, 8A and 8B. The ends of the telescopic rods are connected for example to the respective openings by connection means of the type comprising a nipple and a nozzle, which are known per se. A connection means or hydraulic connection 10 of this type from the prior art is shown in FIG. 2; it comprises a nozzle 12 at the end of a tubular rod which is supported against an axially open nipple 14. The nipple is rigidly connected, by being joined by screwing for example, to a tubular recess which is made in the axis of the opening (not shown) and which is located in the collar or even in the movable portion of the actuator. The recess is not shown here. The two elements of the connection, the nozzle and the nipple 12 and 14, are kept assembled by a nut (not shown) which is caught between the span 12P, which is perpendicular to the axis, and present on the nozzle, and the screw pitch 14V machined on the nipple 14. This type of assembly does not allow any angular displacement between the part by which the fluid is distributed and the tubular rod which is connected thereto. It is rigid.
The following problem may arise from the configurations of the prior art. During the return of the movable portion 32 of the linear actuator towards the casing 1 where the supply of hydraulic fluid is located and in particular when the actuator is at a high pressure, the telescopic system of rods can become locked and buckle under the force of the actuator. The locking occurs if the alignment is incorrect.
The locking, followed by the failure of the system of telescopic rods, can occur during the misalignment between the casing 1 on which the oil inflow is located and the fixed portion 31 of the actuator 3, the portion being rigidly connected to the casing. The misalignment is expected when the turbine engine is subjected to high mechanical stresses.
In the case of a misalignment between the casing and the movable portion of the actuator, an angle is induced between the two telescopic rods which slide one inside the other. The force which is perpendicular to the surface of the rod and the resistance to the sliding increase. If this angle becomes too great and reaches a critical value, depending on the geometric characteristics of the rods and the coefficient of friction between the rods, the friction between the rods can lead to locking. Over-center locking thus occurs and, under the effect of the hydraulic actuator, the rods buckle. The consequence would thus be a failure of the whole system, leading to an extremely critical situation in the case of an aeronautical application.
The over-center locking phenomenon is produced in a mechanical system when the configuration of the system is such that the adhesion prevents any movement, regardless of the strength of the external mechanical influences.